Field of the Invention
The present invention is generally related to electrodes, for example, physical vapor deposited components for electrodes such as those found in biosensors. More particularly, the present invention is related to electrodes formed with non-noble metal alloys, for example, those found in biosensor components.
Description of the Related Art
Biosensors for use in analyzing biological samples are becoming increasingly prevalent. For example, with the rise in cases of diabetes in the world's population, the need for biosensors for measuring blood glucose has risen dramatically. Such biosensors are generally known as glucometers and operate by having a user place a drop of blood on a test-strip associated with the glucometer. The test-strip is configured to be reactive to the amount of glucose in the drop of blood, such that the glucometer can detect and display a glucose level of the user's blood.
The test-strips for glucometer-type biosensors are generally formed with two or more electrodes (e.g., a working electrode and a counter electrode) formed on a substrate. In addition, an enzyme (e.g., glucose oxidase, glucose dehydrogenase, or the like) and a mediator (e.g., ferricyanide, ruthenium complexes, osmium complexes, quinones, phenothiazines, phenoxazines, or the like) will be formed on the working electrode. In operation, a drop of blood will be applied to a test-strip. Thereafter, an electrochemical reaction proportional to the amount of glucose in the blood will take place on the working electrode. In more detail, glucose first reacts with the enzyme (glucose oxidase, glucose dehyrogenase, or the like) and sometimes an enzyme cofactor (PQQ, FAD, or the like) and is oxidized to gluconic acid. The enzyme, cofactor, or enzyme-cofactor complex is temporarily reduced by two electrons transferred from glucose to the enzyme, cofactor, or enzyme-cofactor complex. Next, the reduced enzyme, cofactor, or enzyme-cofactor complex reacts with the mediator, transferring a single electron to each of two mediator species (molecules or complexes), in the case of a mediator that is reduced in a one-electron process. When the mediator species are reduced, the enzyme, cofactor, or enzyme-cofactor complex is thus returned to its original oxidation state. Then, the reduced mediators diffuse to the electrode surface where a pre-determined and sufficiently oxidizing potential is applied to the biosensor so that the reduced mediators are oxidized back to their original oxidation state. The current that is generated by the oxidation of the mediator species by the biosensor is measured and related proportionally to the amount of glucose in the blood.
The quality of the working electrode plays an important role in an accurate measurement of the glucose level of the blood. Specifically, the reproducibility of the electroactive surface area of the electrode, the lot-to-lot repeatability of the electron transfer kinetics of the electrode in a particular glucose measurement arrangement, and long term stability of the electrode material while in storage so that the electrochemical signal that arises from the electrode when the assay is in operation are all factors that lead to improved accuracy of blood glucose test strips. Particularly, it is important that the electrical signals resulting from the electro-activity of the electrode is minimized to prevent bias or noise in the measurement and analysis of biological samples. Typically, this is accomplished by using electrode materials that are intrinsically thermodynamically noble, such as gold, palladium, platinum, iridium, and the like. As such, most current glucometers use electrodes formed from substrates coated with palladium, gold, or other noble metals, generally in the purest form commercially feasible, to function as the working electrode, and for ease of manufacturing, often for the counter electrode or a combined counter and reference electrode. Such noble metals are minimally reactive with interfering substances, and as a result, offer enhanced chemical resistance for consistent and accurate measurements. However, the cost of using such noble metals in electrodes can be prohibitive.
There have been some attempts to use electrodes formed with non-noble metals, so as to reduce manufacturing costs of biosensors. However, such non-noble metal electrodes will generally have an electrochemical response (e.g., dose-responses) that deviates significantly from the electrochemical response of electrodes formed with noble metals. As such, electrodes formed with non-noble metals are generally inadequate for use as direct replacements for noble metals in test-strips for many types of biosensors.
Accordingly, there is a need for an electrode which can be that provides consistent and accurate measurements, while providing a cost effective alternative to the use of noble metals, for example, in biosensors. In particular, there is a need for an electrode formed form a non-noble metal alloy that can be used in a biosensor component to consistently and accurately measure biological samples.